Heart failure afflicts a large portion of the population and results-at least in part-from the inability of sufficient myocardial regeneration in the injured heart. Typically most adult cardiac myocytes (ACMs) remain in a quiescent cell cycle state without signs of proliferation. However, mounting evidence demonstrates limited but measurable new myocytes formation, which is augmented in stressed and injured hearts. Nonetheless, the cellular and molecular mechanism underlying this endogenous ACM renewal remained largely unknown, and current interventions are not sufficient to regenerate the lost heart muscle cells. This application is built on our recent findings that ACMs can dedifferentiate and re- enter the cell cycle for proliferation in prolonged culture or in post-infarct hearts. Histne epigenetic remodeling, such as acetylation or methylation of histone proteins at their lysine (K) residues, is a promising new target for cancer therapies, but is poorly understood in regulating myocyte cell cycle, particularly in adult and injured hearts. We demonstrate that during ACM dedifferentiation and proliferation, there are dynamic post-translational modifications of histone proteins regulated by histone modification enzymes. We found a profound increase of H3K18ac in dedifferentiated/ proliferative myocytes, and that cardiac maturation and cell cycle genes are regulated by H3K18ac modification. The proposed studies are designed to test the hypothesis that epigenetic reprogramming of cardiac, cell cycle and transcription factor genes mediated by H3K18ac is required for their re-expression that leads to cardiomyocyte dedifferentiation and proliferation. Empowered by innovative multi-reporter triple-transgenic mouse models featuring superior myocyte genetic fate tracking and nuclear visualization, the proposed study will define a novel paradigm of mammalian myocyte renewal processes in both healthy and injured hearts. The Specific Aims are: 1) Determine the histone modification states characteristic to myocyte dedifferentiation and proliferation, and the specific histone modification enzymes leading to H3K18ac augmentation. 2) Determine the histone epigenetic regulation by identifying cardiac, cell cycle and transcription factor gene targets of H3K18ac modification during the dedifferentiation/proliferation of ACMs. 3) Test if enhancing H3K18ac histone modification can promote ACM proliferation after myocardial injury. The ultimate goal is to apply the new theories in cardiobiology, i.e. cardiac epigenetics, to develop novel and effective therapy strategies towards cardiac regeneration.